build, imagebuilder: Do not require libncurses-dev

The buildroot and SDK both require `libncurses-dev` to be installed on
the system, however the ImageBuilder uses precompiled binaries.

This patch changes the prerequirements checks to skip the
`libncurses-dev` part if running as ImageBuilder.

Signed-off-by: Paul Spooren <mail@aparcar.org>
(cherry picked from commit 4a1a58a3e2d2360950ff23a05f1f55b779174d6a)

include/prereq-build.mk

@@ -51,10 +51,12 @@ $(eval $(call TestHostCommand,working-g++, \
 		g++ -x c++ -o $(TMP_DIR)/a.out - -lstdc++ && \
 		$(TMP_DIR)/a.out))
 
+ifndef IB
 $(eval $(call TestHostCommand,ncurses, \
 	Please install ncurses. (Missing libncurses.so or ncurses.h), \
 	echo 'int main(int argc, char **argv) { initscr(); return 0; }' | \
 		gcc -include ncurses.h -x c -o $(TMP_DIR)/a.out - -lncurses))
+endif
 
 ifeq ($(HOST_OS),Linux)
   zlib_link_flags := -Wl,-Bstatic -lz -Wl,-Bdynamic